You should start the client in logging mode (e.g. clarify.exe
-sqllog<path-and-filename> -sqlhint), then perform the "switch user" and
examine the log filename you specified.

While you may not have custom code on the switch user form, you may still
have custom code that doesn't take a proxy user into account on the case
forms or other forms that may be used when you are acting as another user
(e.g. a proxy.)

Post the pertinent sections of your log file, and I will try to assist you
further.